Shear's Cysts of the Oral and Maxillofacial Regions. Paul M. Speight
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In the study of Lustmann and Shear (1985 ), 23 personally observed cases were reported. The patients' ages ranged from 4 to 12 years, with one exceptional case aged 19 years. The M : F ratio was 1.6 : 1. The mandible was affected more frequently than the maxilla and the deciduous molars were the teeth most often involved. In 9 cases buccal expansion was noticed and in 8 cases the permanent tooth buds were displaced. Mass et al. (1995 ) gave a detailed account of 36 cases, of which 22 (61%) were associated with mandibular molars. They also noted that the lesions are not associated with the apices of the teeth, but usually present as a periradicular radiolucency overlying the crowns of the unerupted permanent premolars. This makes diagnosis uncertain, since it may be difficult to differentiate a deciduous radicular cyst from a hyperplastic follicle or even a dentigerous cyst of the permanent successor, which in this context may be inflammatory in origin (the inflammatory dentigerous cyst is discussed in Chapter 5).
Residual Cyst
Residual radicular cysts are those that remain in the jaws after removal of the cause of the lesion – the offending non‐vital tooth. Although the reasons for the persistence of cysts are contentious, there have been relatively few publications on the subject and the whole notion of the existence of such cysts has been challenged on the basis that a persistent radiolucency after removal of the offending tooth may merely represent an ongoing healing process (Walton 1996 ; Lee et al. 2014 ). High and Hirschmann (1986 , 1988 ) studied a series of asymptomatic and symptomatic residual cysts. They were interested in the factors that decide whether a radicular cyst will resolve or persist after tooth removal and the natural history and behaviour of these cysts once established. With regard to the asymptomatic cysts, they showed that there was a decrease in size with increasing duration of the cyst (r = 0.5; P < 0.005; High and Hirschmann 1986 ). There was an unexpectedly large number in the mandibular premolar region and there was a direct relationship between the age of the cyst and the radiological and histological evidence of mineralisation (P < 0.001). There was an overall reduction in epithelial thickness with cyst age and all cysts showed minimal chronic inflammatory changes. Their results support the notion that the vast majority of residual cysts are slowly resolving lesions. Nevertheless, they do persist and the authors did not provide evidence of complete healing.
In their second paper, High and Hirschmann (1988 ) showed that symptomatic cysts were larger than asymptomatic lesions, and that the negative correlation of size with cyst duration was not as strong (r = 0.39; P < 0.05). Cyst ages varied from 1 month to 20 years and there was again a perplexingly high frequency in the mandibular premolar region. Acute and chronic inflammatory cell infiltration showed variable intensity and there was an inverse relationship between the presence of acute inflammation and cortication of the cyst wall radiographically (P < 0.001). There were no obvious causes for the inflammation in deeply positioned residual cysts. The authors suggested that in symptomatic cases chronic inflammation could persist and gradually worsen, and that acute inflammatory episodes may be triggered by release of pro‐inflammatory factors from dead or dying cells.
Nair (1998 , 2003 , 2006 ) considered that the type of cyst was important with regard to persistence after treatment. Although he discussed non‐healing cysts after endodontic treatment, his conclusions are relevant to residual cysts. He confirmed the work of Simon (1980 ), who showed that there were two types of radicular cyst. These are discussed in more detail later in this chapter but, to summarise, there is the true radicular cyst, which contains a closed cavity entirely lined by epithelium, and the periapical pocket cyst (also called bay cyst), in which the epithelium is attached to the margins of the apical foramen in such a way that the cyst lumen is essentially a pouch or pocket, which communicates directly with the affected root canal. Thus, it is expected that the pocket cyst would heal after treatment or tooth extraction, while the true cyst, being completely enclosed, is ‘self‐sustaining’ and may therefore persist in the absence of the cause. Nair et al. (1996 ) suggested that only 15% of periapical lesions were radicular cysts, but of these 61% were true cysts and 39% were pocket cysts. If only true cysts persisted after removal of the offending tooth, this may account for the relatively low frequency of residual cysts.
Regardless of the finer details of the biology of these lesions, from the clinical point of view it is evident that a radiolucent lesion may persist and may produce symptoms, for a substantial period of time after extraction of the offending tooth. On biopsy, many show the typical features of a cyst and residual cyst remains an appropriate term for these lesions.
Radiological Features
The classic description of the radiological appearance of radicular cysts is that they are round or ovoid radiolucencies surrounded by a narrow, radiopaque or corticated margin that extends from the lamina dura of the involved tooth (Figure 3.4; see also Figure 2.2). In infected or rapidly enlarging cysts, the corticated margin may not be present. A residual cyst is usually round to oval with a well‐demarcated and often corticated margin. They are found within an edentulous area of the jaws, at a site of a previous tooth extraction (Figure 3.5). With a residual cyst, the differential diagnosis of keratocyst must be considered. A radicular cyst on the lateral margin of a root in association with an accessory root canal must be differentiated from a lateral periodontal cyst. Root resorption is not often seen on routine radiographs, but it may occur.
Despite these well‐described features, many studies have shown that it is not possible to reliably differentiate radiologically between a radicular cyst and a periapical granuloma. In one of the largest studies, which has not been repeated, Mortensen et al. (1970 ) examined histological material of 396 periapical lesions with a diameter of 5 mm or more, that had been classified pre‐operatively as cysts or granulomas on radiological evidence. A correct preliminary diagnosis had been made in 81% of 232 granulomas, but in only 48% of 164 cysts. They also showed that the relative number of granulomas decreased with increasing size of the lesion, whereas the relative number of cysts increased. Table 3.1 summarises their data and shows that if a lesion measured between 10 and 14 mm in radiographic diameter, there was an equal chance of it being a granuloma or a cyst. About one‐third of lesions measuring 5–9 mm were cysts and one‐third of lesions measuring 15 mm or more were granulomas. However, they found that very large lesions, over 20 mm, were almost always cysts, a finding confirmed by others (Natkin et al. 1984 ).
Figure 3.4 Radiograph of a radicular cyst. The lesion is a well‐defined radiolucency associated with the apex of a non‐vital root‐filled tooth.
Mortensen et al. (1970 ) also noted that many cysts had a diffuse radiographic margin and therefore lacked the cortication often described as typical for radicular cysts. This suggested that a corticated margin, although common, was not a specific diagnostic criterion for a cyst, a finding supported by others. Ricucci et al. (2006a ) examined 57 periapical radiolucent lesions and found that of 10 lesions with a corticated outline, only 3 were cysts. Conversely, 40 of the 47 lesions without cortication were granulomas.